1. Field of the Invention
The present invention relates to a method of preparing greases, and in particular greases thickened with thickeners having urea functional groups. More specifically, the present invention relates to a method of preparing greases using high pressure and high flow rate impingement for effecting the mixing of the grease and the reaction to form the thickeners.
2. Description of the Related Art
Grease manufacturing technologies have not changed significantly over the last decade. The current capabilities center around the use of standard kettle procedures, batch processing. New manufacturing techniques for greases to help reduce the complexity of synthesis of grease formulas are needed. More effective and efficient manufacturing processes are always desired, particularly if the new process also imparts desired physical properties into the grease formulas. One such important property is “noise”.
The quiet running properties (noise) of greases used to lubricate deep groove ball bearings have become increasingly important to bearing manufacturers in their selection of factory fill greases. Historically, bearing manufacturers became increasingly concerned about bearing vibration that manifested itself as audible sound as the demand grew for quieter machines. As bearings were machined to finer tolerances, becoming inherently less noisy, the noise contributions of the greases used to lubricate them became increasingly apparent. Consequently, the major bearing manufacturers independently developed instrumentation that allowed measurement of the contribution of grease to bearing noise. In addition, correlation of bearing life to the presence of contaminants promoted an even greater concern with grease noise testing because the assumption is often made that grease noise always correlates to the presence of contaminants and therefore with shortened bearing life. Although most grease manufacturers would agree that knowing the noise characteristics of a grease does not provide sufficient information to allow prediction of the life of a bearing lubricated with it, noise testing is nonetheless increasingly used to assess the overall quality of ball bearing greases. Grease manufacturers therefore must be concerned with the noise quality of their products and with the various methods by which grease noise quality is determined if they are to continue to supply greases to the bearing manufacturing industry.
Although grease noise testing has been the subject of numerous publications over the past twenty-six years, no standard test instrument, test bearing, or test protocol has been adopted by either grease suppliers or bearing manufacturers during this time. In fact, a wide variety of proprietary grease noise testing methods is currently in use, particularly in the bearing manufacturing industry, where each major bearing manufacturer has developed its own proprietary instrumentation and methods. In addition, each method is considered by its proponents to provide a competitive edge for the company that uses it.
Because of the above considerations, testing the quiet running (noise) properties of grease has been an issue. Originally, a manual test was developed which allowed assessment of the running properties of a batch of grease by the feel of a bearing packed with it. As the noise quality of bearings themselves improved, it became necessary to be able to detect lower and lower levels of bearing vibration. As a result, Chevron Research (Richmond, Calif.) began using a modified bearing vibration level tester (an anderonmeter) to test for grease noise and began carefully studying the effects of additives and processing variables on grease noise. The anderonmeter, which was originally developed to assess bearing vibrational quality, measures the radial displacement of the outer race of a bearing as a function of its rotation. In fact, the name anderon is an acronym for “angular derivative of the radial displacement”. In physical terms, the anderon is expressed as displacement distance/unit rotation:
The sensor head, which is in contact with the outer race, detects bearing vibration. The sensor signals are amplified and filtered into three frequency bands which span the range of audible sound frequencies:                Low: 50-300 Hz        Medium: 300-1,800 Hz        High: 1,800-10,000 Hz.        
Vibration (noise) due to grease can be detected in the medium and high frequency bands. In the earliest version of the Chevron grease noise test, the highest recorded vibrational spike recorded in the medium band during a one-minute run was averaged for five bearings and the average reported as the grease anderon value.
Chevron later refined its test instrument, adding noise pulse counting capability. The pulse counter allows the detection of transients, which are too fast to be recorded on the strip chart recorder. During a test the signal level in each band is displayed on a corresponding meter and is recorded on a strip chart recorder, while the pulse counter detects and displays a figure proportional to the number of vibrational transients that occur above a preset threshold amplitude level. At the end of each test run, the medium band pulse counter reading is noted and the strip chart record of the medium band signal is examined. The first five seconds on the chart are disregarded as start-up noise and the highest amplitude peak (spike) anderon value recorded during the remaining 55 seconds is noted. The noted results for five bearings are averaged and reported as anderon spike value/pulse count.
Different grease compositions have an impact on the amount of bearing vibration and audible noise. Grease noise is attributed to the presence of particles in grease. There are process techniques to help control the particle size during grease manufacture, but better techniques to further improve the noise properties is still desired.
Grease compositions containing a variety of gellant thickeners with urea functional groups have been developed. The polyurea reaction is preferably carried out in situ in the grease carrier, and the reaction product may be utilized directly as a grease.
The search continues for new effective and efficient manufacturing processes for greases. Particular benefits would be realized if such a process also produces a low noise grease, especially a polyurea type grease.